Sort out a bit the pretyping of universes.
Sort out a bit the pretyping of universes.

We clearly delineate levels, universes and sorts. In addition we also forbid
dubious syntax like Prop + 1 and SProp + 1 that was previously allowed but
interpreted with a quotient by the kernel.

Zimmi48/coq,coq/coq,ejgallego/coq,ejgallego/coq,ejgallego/coq,SkySkimmer/coq,ejgallego/coq,SkySkimmer/coq,coq/coq,coq/coq,Zimmi48/coq,coq/coq,Zimmi48/coq,SkySkimmer/coq,Zimmi48/coq,SkySkimmer/coq